The Mechanism and Dynamics of the Generation and Occurrence for Wenchuan MS8.0 earthquake

On 12 May 2008, an MS8.0 earthquake occurred in Wenchuan, Sichuan Province. Its causative fault is the Yingxiu–Beichuan fault. Analysis of the velocity field before the earthquake discovers that the shortening rate along the direction of N103°E from west Bayan Har block (BHB) to the Longmen Shan fault zone (LFZ) is 13.0 mm/a, the right lateral slipping rate of LFZ is 1.1 mm/a, and the faults are locked. The coseismic displacement field shows that the earthquake maybe was caused by the thrust of BHB in direction of SE and the underthrust of Sichuan Basin (SB) in the direction at NW simultaneously. Analysis of the strain field shows that the principal compressive and the principal tensile strain rates in the epicenter area are –30.840×10–9/a and 13.956×10–9/a, respectively before the earthquake, and the principal compressive strain axis is in a direction of N105.4°E which is consistent with the direction of N103°E of the principal compressive stress axis from focal mechanism results. The earthquake fault trend and slipping direction obtained by the mechanism proposed in the paper are also consistent with the results of the focal mechanism and earthquake field investigations. The interactions of the Indian, the Philippine Sea and the Pacific plates with the Eurasian plate are long-term acting forces responsible for the Wenchuan great earthquake occurrence. The Sumatra-Andaman huge earthquake on December 2004 promoted occurrence of the Wenchuan earthquake.